Power control is an important radio resource management method in radio communication systems. Use of excessive power decreases battery life and increases co-channel interference. Power control aims to control the transmission power levels in such a way that acceptable quality of service for the users is guaranteed with lowest possible transmission powers.
Types of power control include Open Loop, Outer Loop and Closed Loop. Open Loop power control is used in the beginning of a radio link connection establishment to set the transmission power according to measurements of the return channel link gain. Outer loop power control sets the target signal-to-interference ratio (SIR) to such a level that sufficient quality of service is guaranteed. Closed loop power control aims to keep the receiver SIR at this target value using feedback signals from the receiver. For example, adaptive, closed loop power control algorithms have been proposed for CDMA (Code Division Multiple Access) cellular communication systems.
In closed-loop power control, the base station (BS) (also known as a base radio or repeater) measures the fading effects in the signal received from each mobile station by measuring the signal power and the bit-error rate (BER). Typically, the received power is measured by averaging multiple samples of the received sequence, while the BER is computed by comparing the received sequence with a predetermined transmitted sequence. The base station then compares these quantities with a reference point. Based on this comparison, the base station transmits a one-bit signal, known as the power bit, to each mobile station (MS) (also known as a subscriber unit), commanding it to either increase or decrease its power by a fixed amount, e.g., 1, 0.5, or 0.25 dB. For example, the power bit rate is 800 Hz in IS-95 standards and 1500 Hz in 3G WCDMA standards.
A problem in closed-loop power control is that extensive control signaling consumes radio channel bandwidth. Further, most closed-loop power control algorithms assume a full-duplex mobile station (MS) in order to adjust transmission power during a transmission. With a half-duplex MS, closed-loop power control cannot be achieved during a transmission without introducing periods of time where the MS stops transmitting on the up-link in order to receive feedback on the down-link.